I. Field of the Invention
The present invention relates to the field of separation of catalysts and sorbents, generally classified in U.S. Patent Class 208, subclass 120.
In conventional fluid bed cracking of hydrocarbon feedstocks, it is the practice, because of the rapid loss in catalyst activity and selectivity, to add fresh catalyst continuously or periodically, usually daily, to an equilibrium mixture of catalyst particles circulating in the system. If metals, such as nickel and vanadium, are present in the feedstock, they accumulate almost completely on the catalyst, thus drastically reducing its activity, producing more undesirable coke and hydrogen, and reducing selective conversion to gasoline. In such cases, catalyst replacement additions may have to rise significantly.
Fluid cracking catalysts generally consist of small microspherical particles varying in size from 10 to 150 microns and represent a highly dispersed mixture of catalyst particles, some present in the unit for as little as one day, others there for as long as 60-90 days or more. Because these particles are so small, no process has been available to remove old catalysts from new. Therefore, it is customary to withdraw 1 to 10% or more of the equilibrium catalyst containing all of these variously aged particles, just prior to addition of fresh catalyst particles, thus providing room for the incoming fresh "makeup" catalyst. Unfortunately, the equilibrium catalyst withdrawn itself contains, 1-10% of the catalyst added 2 days ago, 1-10% of the catalyst added 3 days ago, and so forth. Therefore, unfortunately a large proportion of the withdrawn catalyst represents very active catalyst, which is wasted.
Catalyst consumption can be high. The cost associated therewith, especially when high nickel and vanadium are present in any amount greater than, for example, 0.1 ppm in the feedstock can, therefore, be great. Depending on the level of metal content in feed and desired catalyst activity, tons of catalyst must be added daily. For example, the cost of a ton of catalyst at the point of introduction to the unit can be $2,000 or more. As a result, a unit consuming 20 tons/day of "makeup" catalyst would require expenditures each day of $40,000. For a unit processing 40,000 barrels/day (B/D) this would represent a processing cost of $1/B or 2.5 cents/gallon, for makeup catalyst cost alone.
In addition to "makeup" catalyst costs, an aged high nickel and vanadium-laden catalyst can also reduce yield of preferred liquid fuel products, such as gasoline and diesel fuel, and instead, produce more undesirable, less valuable products, such as dry gas and coke. Nickel and vanadium on catalyst also accelerate catalyst deactivation, thus further reducing operating profits, and reducing throughput capacity of the conversion unit.
II. Description of the Prior Art
Patents related to processing metal-laden catalyst feedstocks and involving magnetic separation, classification and attrition include U.S. Pat. No. 4,359,379 and U.S. Pat. No. 4,482,450 to Ushio.
"Magnetic Methods For The Treatment of Materials" by J. Svovoda published by Elsevier Science Publishing Company, Inc., New York (ISBNO-44-42811-9) Volume 8) discloses both theoretical equations describing separation by means of magnetic forces with the corresponding types of equipment that may be so employed. Specific reference at pages 135-137 is made to cross-belt magnetic separators and pages 144-149 refer to belt magnetic separators involving a permanent magnet roll separator, as well as pages 161-197 which refer to high gradient magnetic separators, all of which are efficient in separating magnetic particles. Svovoda teaches a number of magnetic separation techniques useful with this invention, including the preferred RERMS, HGMS and the drum-roller device.
Magnetic separation of catalyst is covered in U.S. Pat. No. 4,406,773 (1983) of W. P. Hettinger et.al, which prefers use of a high gradient magnetic field separator (HGMS) or a carrousel magnetic separator which uses multiple HGMS units to achieve selective separation.